Rim Fire near Yosemite National Park 70% contained

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From Dave Alsup and Tina Burnside

(CNN) -- The stubborn Rim Fire, one of the largest wildfire in California's history, is 70% contained, the U.S. Forest Service said Monday night.

With rain and cooler temperatures in the forecast, firefighters who have toiled tirelessly for days will likely make further gains in the coming days.

Still, officials don't expect full containment until October 20.

The Rim Fire started August 17 and swallowed more than 235,000 acres -- making it the fifth-largest wildfire in California history. It has cost the state more than $39 million to date.

Although the fire has consumed tens of thousands of acres inside Yosemite National Park, it has so far had little or no direct impact on Yosemite Valley, a popular spot for tourists and home to many of the park's iconic attractions, including the El Capitan rock formation.

Authorities don't know what started the Rim Fire -- although the fire chief in the town of Twain Harte said illegal marijuana growers could be the unintentional culprits.

"It might be some sort of illicit grove, marijuana grow-type thing." Todd McNeal told a meeting on August 23. The video of the meeting was posted on YouTube and picked up steam recently.

Calls to the Twain Harte fire department were not answered Monday night. But Mark Healey with the Rim Fire Information Line dismissed the suggestion, calling it "rumors."